In radar and communications systems, it is important to discriminate the in-range target from the clutter return of out-of-range unwanted signals. Pseudo-random noise (PN) coded systems are used to obtain range information while using a low power continuous wave (CW) transmitter.
PN code sequences, sometimes referred to as maximal length binary code sequences have approximately the same statistical properties as randomly generated binary code sequences and are referred to in the literature by various names depending on their use.
Most applications of PN code sequences modulate the PN code onto a carrier frequency prior to transmission, and subsequently demodulate the PN code in the receive path in a correlation process. Phase modulation is commonly used in PN code applications in which the two binary states of the PN code sequence effect a phase shift of the carrier separated by 180 degrees.
In designing a PN coded system (radar or communications) there is a tradeoff to be made between PN code pulse width, PN code length, Doppler frequency band width and transmitter center frequency. The PN code pulse width establishes the system's range resolution, the PN code length establishes the system's out-of-range rejection of returns from unwanted clutter signals and the Doppler frequency band width establishes the maximum closing velocity. For a given range resolution, closing velocity and transmitter center frequency, the PN code is limited in length and therefore the rejection of returns from unwanted clutter signals is limited.
Conventional PN systems use bi-phase modulation where the two phase states are precisely out-of-phase. The rejection of undesired signals in such systems is proportional to the square of the PN code length. Thus the amount of rejection is theoretically limited when the PN code is limited in length. For example, in the case of a radar fuze, the limited rejection of clutter and interference sets a lower bound on the sensitivity of the fuze.
Thus what is needed are a method and apparatus to reduce the range sidelobes beyond the theoretical limit to improve auto-correlation and out-of-range clutter rejection.